1. Place onion, celery, potatoes and carrots in bottom of slow cooker, lay brisket on top.
2. Whisk together the package of spices that comes with the brisket, 1 cup broth, bay leaf, garlic, Worcestershire sauce and dry mustard.
3. Pour over brisket, cover pot.
4. Cook on low setting for 8 to 10 hours, adding cabbage wedges for the last hour of cooking.
5. Remove the beef and vegetables to a platter and cover to keep warm.
6. Strain the cooking liquid through a wire sieve into a small saucepan and bring to a boil. Blend the cornstarch into the remaining cup of beef broth until smooth, then pour into the boiling cooking liquid. Whisk constantly until thickened.
7. To serve, slice meat onto hot serving plates, accompany with the cooked potatoes, carrots, celery, cabbage wedges the thickened sauce.